The present invention is generally directed to a trimming circuit for permanently setting a circuit parameter, such as a voltage, to a desired level. The present invention is more particularly directed to such a circuit wherein the circuit parameter may be temporarily set to an adjusted level to enable the determination as to whether the adjusted level is the desired level before the circuit parameter is permanently set.
There are many applications where it is necessary to externally adjust a circuit parameter of a circuit in order to provide a desired circuit parameter value. One such application is in the field of integrated circuits wherein it is necessary to externally adjust a particular parameter value, such as a voltage level, to derive bias or reference voltages of particular levels. In such applications, the integrated circuit generally includes a voltage divider including a plurality of serially coupled resistors with voltage taps between the resistors and a trimming circuit associated with at least some of the taps. The trimming circuits are responsive to externally applied voltages for selecting one of the taps to provide a voltage from the voltage divider of a desired level. The externally applied voltages are used to open circuit a fuse within each trimming circuit and, once a fuse is open circuited, the trimming circuit causes the resistor divider tap associated with that fuse to be permanently selected. Through this process, the voltage can be adjusted to a particular value.
While such arrangements for trimming voltage levels has been generally successful, there has been a continual problem in that it has not been known what effect the tap selection would have on the overall circuit until after the particular tap has been selected. This has been caused by the resistance values of the voltage dividers not being exactly the values anticipated and the adjustment process not being irreversible. Once a fuse has been converted from a low resistance condition to an open circuit, high resistance, condition, it cannot thereafter be re-converted to a low resistance condition. As a result, the trimming of such circuit parameters has been a "blind" operation with the potential for a resulting voltage level not being at the desired required level. This can cause an integrated circuit to operate out of certain required specifications resulting in the reject of the integrated circuit with no avenue for correcting the problem.
It is therefore a general object of the present invention to provide a new and improved trimming circuit for adjusting the value of a circuit parameter.
It is a more particular object of the present invention to provide such a trimming circuit wherein a voltage level may be temporarily set to enable the determination of the appropriateness of permanently and irreversibly setting the voltage level.
It is a still further object of the present invention to provide such a trimming circuit which may be implemented in an integrated circuit.